Method and apparatus for implementing accurate angle screens

Abstract

The present invention is a method and apparatus for processing a color document for moire-free printing, the color document including a plurality of color separations each defined with a set of image signals representing optical density with m possible density levels, while a printer is capable of rendering density with n density levels. The invention employs a spatial resolution scaler to produce m-bit signals at a spatial resolution that is a function of the halftone screen angle, base dot pitch, and desired halftone cell size. Subsequently, the m-bit signals at the desired spatial resolution (frequency and angle) are halftoned to produce n-bit signals that are printed with a raster output scanning-type printer suitable for printing at selectively variable spots per inch in both the fast and slow scan directions.

I claim: 
     
       1. A color document printing system for eliminating moire in printed output, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, comprising: a printer, capable of rendering density with n density levels, adapted to print the color separations at distinct spatial resolutions, so that when the color separations are superposed on a substrate, a selected color is defined;   a source of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation;   a spatial resolution scaler, operatively connected to said source of image signals;   a first halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in a subset of the color separations to a number of levels n representing optical density, said processor generating a first periodic pattern at a first spatial resolution;   a second halftone processor, operatively connected to said spatial resolution scaler, for reducing the number of levels m representing optical density in at least one remaining color separation to a number of levels n representing optical density, said processor generating a second periodic pattern at a second spatial resolution; and   a video processor directing the distinct spatial resolution signals processed at each halftone processor to said printer to print the processed color image.   
     
     
       2. The color document printing system of claim 1, wherein the number of color separations is at least three. 
     
     
       3. The color document printing system of claim 1, wherein said spatial resolution scaler comprises: color separation identification means for determining which color separation is being received from said source;   spatial resolution selection means, responsive to the color separation received, for selecting spatial resolution pairs associated with the color separations; and   two-dimensional scaling means for scaling the image signals of each color separation in accordance with the first and second spatial resolutions selected associated with the color separation.   
     
     
       4. The color document printing system of claim 1, further comprising spatial resolution determining means for predefining the spatial resolution pair for each color separation based upon a selected screen angle, a base dot pitch, and a desired halftone cell size. 
     
     
       5. The color document printing system of claim 4, wherein said spatial resolution determining means comprises: an arithmetic processing unit for receiving the selected screen angle, the base dot pitch, and the desired halftone cell size and calculating desired spatial resolution pair; and   resolution memory for storing a plurality of desired spatial resolution pairs.   
     
     
       6. The color document printing system of claim 1, wherein said printer comprises a focused ion stream printer. 
     
     
       7. The color document printing system of claim 6, wherein said focused ion stream printer includes a plurality of apertures, each of said apertures being controlled in response to one of the distinct spatial resolution signals processed by one of said halftone processors. 
     
     
       8. The color document printing system of claim 1, wherein said printer comprises a polygon-based raster output scanner. 
     
     
       9. The color document printing system of claim 8, wherein said polygon-based raster output scanner is a multiple-pass scanner including a variable speed polygon motor operating in conjunction with a variable pixel frequency video driver. 
     
     
       10. The color document printing system of claim 8, wherein said polygon-based raster output scanner is a multiple-beam scanner including: a photoreceptor; and   a plurality of polygons, each operating at different speeds, each polygon positioned so as to direct a beam to expose the photoreceptor at a unique exposure location.   
     
     
       11. A method for preparing a color document for moire-free printing, the color document including a plurality of color separations wherein each separation is defined with a set of image signals representing optical density with m possible density levels, while a printer is capable of rendering density with n density levels, comprising: receiving a set of image signals describing the color document with a plurality of color separations, each image signal representing optical density as one of m levels for a discrete area of a color separation;   spatially scaling the image signals of a first and second color separation;   halftoning the m level image signals of the first color separation to n level image signals representing optical density in a manner generating a first periodic pattern having a fixed frequency and a distinct first angle for the first separation;   halftoning the m level image signals of a second color separation to n level image signals representing optical density in a manner generating a second periodic pattern having the fixed frequency and a distinct second angle for the second separation; and   directing the first and second periodic patterns to said printer to print the processed color image.   
     
     
       12. The method of claim 11, wherein the step of receiving a set of image signals includes receiving at least three distinct color separations. 
     
     
       13. The method of claim 11, wherein said step of spatially scaling the image signals comprises: determining which color separation is being received from said source;   selecting a spatial resolution pair associated with the color separation being received; and   scaling the image signals of the color separation in accordance with the spatial resolution associated with the color separation.   
     
     
       14. The method of claim 11, further comprising the step of predefining a spatial resolution pair for each color separation based upon a selected screen angle, a base dot pitch, and a desired halftone cell size. 
     
     
       15. The method of claim 14, wherein said step of predefining a spatial resolution pair comprises: receiving the selected screen angle (α), the base dot pitch (L), and the desired halftone cell size (n);   calculating desired spatial resolution pair values; and   storing the desired spatial resolution pair values in memory.   
     
     
       16. The method of claim 15, wherein said step of calculating desired spatial resolution pair values comprises: determining the desired x-direction spatial resolution in accordance with the formula, x=[(L cos α)/n] -1  ; and   determining the desired y-direction spatial resolution in accordance with the formula, y=[(L sin α)/n] -1 .